Five of the most underrated engines and their true power

Find out how much power a 426 Hemi actually makes. And how much did Ford underrate the GT?

by
Clayton Seams | July 24, 2014

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You might think that it’s advantageous for an automaker to always claim the highest possible output they can on their engines. But sometimes, in order to get lower insurance rates or sneak into certain racing classes, manufacturers underrate their engines and say they make less power than they actually do. Naturally, this leads to a lot of talk about what engines “really make.” Well, the talk can stop here because we’ve gathered the facts on five of the most notoriously underrated engines of all time.

Chevrolet L88 427

This is the meanest “rat motor” of them all. The L88 used a high-overlap cam and a crushing 12.5:1 compression ratio to become one of the most powerful engines of the muscle car era. The engine was laughably underrated at 430 horsepower at 5,200 rpm but GM wasn’t completely dishonest. The engine actually did make 430 horsepower at 5,200 rpm, but GM neglected to mention that its power peak was around 6,500 rpm where it made over 500 horsepower. As can be seen in the video below, a stock recreation of an L88 engine made 557 horsepower. It’s worth noting that the engine in the video doesn’t have parasitic losses like a power steering pump or full exhaust system, but the true output of an L88 at the flywheel when fully dressed is around 510 horsepower.

Is there any engine that has more myth and legend surrounding it? The mystical Hemi is able to conjure wild stories from enthusiasts whenever the name is mentioned. The 426 street Hemi was rated at 425 horsepower from the factory. This Chrysler dyno sheet from decades ago and this close-to-stock dyno session of a new 426 Hemi both put the gross horsepower rating at about 470 horsepower without full exhausts and other engine-driven accessories.

Verdict: Underrated by 45 horsepower

Ford 428 Cobra Jet

Firstly, “Cobra Jet” is the coolest engine name ever. The Cobra Jet was a high-performance version of Ford’s 428 big-block and it was so laughably underrated that even the NHRA ignored Ford’s numbers. The engine was rated by Ford at 335 horsepower, but the NHRA gave it a 390-horsepower rating. Most experts including autojournalist Roger Huntington acknowledge that the true output is about 410 horsepower.

Verdict: Underrated by 75 horsepower

Ford GT 5.4-litre V8

2006 Ford GT.

Don’t think that underrating engines is something unique to the 1960s. The practice of saying your engine is less powerful than it actually is can still be used to full insurance companies. Ford rated the 5.4-litre supercharged V8 in the GT at 550 horsepower at the crank. This dyno test of a stock GT produced 562 horsepower at the wheels. This is a staggering number and other GT owners have reported stock cars producing 520 to 550 horsepower to the wheels. If we factor in a conservative 15% drivetrain loss, that means these engines are actually making around 630 horsepower!

Verdict: Underrated by 80 horsepower

BMW M5 twin turbo V8

2013 BMW M5.

It would seem that the Germans are getting into the underrating game as well. This dyno test of a stock 2013 BMW M5 showed 527 horsepower at the wheels. Factoring in a 15% drivetrain loss puts the approximate flywheel output at 605 horsepower. The M5 is “only” rated at 560 horsepower, so this is quite the jump. It wouldn’t be the first time BMW has underrated one of its engines.

Verdict: Underrated by 45 horsepower

Are there any more engines that are underrated that we didn’t list? Let us know in the comments below.